Methods and apparatus for ion beam angle measurement in two dimensions

ABSTRACT

An angle measurement system for an ion beam includes a flag defining first and second features, wherein the second feature has a variable spacing from the first feature, a mechanism to translate the flag along a translation path so that the flag intercepts at least a portion of the ion beam, and a sensing device to detect the ion beam for different flag positions along the translation path and produce a sensor signal having a first signal component representative of the first feature and a second signal component representative of the second feature. The first and second signal components and corresponding positions of the flag are representative of an angle of the ion beam in a direction orthogonal to the translation path. The sensing device may be a two-dimensional array of beam current sensors. The system may provide measurements of horizontal and vertical beam angles while translating the flag in only one direction.

FIELD OF THE INVENTION

This invention relates to systems and methods for ion implantation and,more particularly, to methods and apparatus for measuring the incidenceangle and/or parallelism of an ion beam in two dimensions.

BACKGROUND OF THE INVENTION

Ion implantation is a standard technique for introducingconductivity—altering impurities into semiconductor wafers. A desiredimpurity material is ionized in an ion source, the ions are acceleratedto form an ion beam of prescribed energy and the ion beam is directed atthe surface of the wafer. The energetic ions in the beam penetrate intothe bulk of the semiconductor material and are embedded into thecrystalline lattice of the semiconductor material to form a region ofdesired conductivity.

Ion implantation systems usually include an ion source for converting agas or a solid material into a well-defined ion beam. The ion beam ismass analyzed to eliminate undesired ion species, is accelerated to adesired energy and is directed onto a target plane. The beam may bedistributed over the target area by beam scanning, by target movement orby a combination of beam scanning and target movement.

In one prior art approach, a high current, broad beam ion implanteremploys a high current density ion source, an analyzing magnet to directa desired species through a resolving slit and an angle corrector magnetto deflect the resulting beam, while rendering the beam parallel anduniform along its width dimension. A ribbon-shaped ion beam is deliveredto a target, and the target is moved perpendicular to the long dimensionof the ribbon beam to distribute the ion beam over the target.

The delivery to a semiconductor wafer of a parallel ion beam at a knownincidence angle is an important requirement in many ion implantationapplications. A parallel ion beam is one which has parallel iontrajectories over the surface of the semiconductor wafer. In cases werethe ion beam is scanned, the scanned beam is required to maintainparallelism over the wafer surface. The parallel ion beam preventschanneling of incident ions in the crystal structure of thesemiconductor wafer or permits uniform channeling in cases wherechanneling is desired. In addition, a parallel ion beam at a knownincidence angle is required in tilted implant applications to ensureuniform results. These requirements have made it necessary to measurebeam parallelism and direction and to adjust these parameters ifnecessary. Techniques for adjusting beam parallelism in ion implantersare disclosed in U.S. Pat. No. 6,437,350, issued Aug. 20, 2002 to Olson,et al.

One known approach to measuring ion beam angle is disclosed in U.S. Pat.No. 6,791,094, issued Sep. 14, 2004 to Olson et al. An object is placedin the ion beam, and the size and relative position of the shadow castby the object is measured. An ion beam incidence angle and beamdivergence monitor is disclosed by Larsen et al. in U.S. PatentPublication No. 2002/0121889 A1, published Sep. 5, 2002. The measurementdevice uses an aperture and a variable resistor to measure implantangle. Both of the disclosed techniques have a limitation in that theyare capable of providing angle information in only one dimension. Movingthe apparatus across the beam permits measurement only in the directionof motion. To make measurements in another direction, an additional ormore complex mechanism is necessary to drive the object or slit in thedesired direction.

Additional techniques for measuring ion beam angle are disclosed in U.S.Pat. No. 5,039,861, issued Aug. 13, 1991 to Swenson; U.S. Pat. No.5,180,918, issued Jan. 19, 1993 to Isobe; and U.S. Pat. No. 5,898,179,issued Apr. 27, 1999 to Smick et al. All of the known prior art ion beamangle measuring techniques have had one or more disadvantages, includinglimited angle measuring capabilities, lack of accuracy and high cost.

Accordingly, there is a need for new and improved methods and apparatusfor measuring ion beam incidence angles.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, an angle measurementsystem for an ion beam is provided. The angle measurement systemcomprises a flag defining a first feature and a second feature, whereinthe second feature has a variable spacing from the first feature as afunction of location on the second feature, a mechanism to translate theflag along a translation path so that the flag intercepts at least aportion of the ion beam, and a sensing device to detect the ion beam fordifferent flag positions along the translation path and to produce inresponse to the detected ion beam a sensor signal having a first signalcomponent representative of the first feature and a second signalcomponent representative of the second feature. The first and secondsignal components and corresponding positions of the flag arerepresentative of an angle of the ion beam in a direction orthogonal tothe translation path.

In various embodiments, at least one of the first and second features ofthe flag comprises a slot in the flag, an outside edge of the flag, oran inside edge of an aperture in the flag. In one embodiment, the firstfeature comprises a slot in the flag and the second feature comprises anangled edge of the flag. The first and second features may be straightand may be oriented at an acute angle.

The sensing device may comprise one or more beam current sensorspositioned to intercept the ion beam. In one embodiment, the sensingdevice comprises a two-dimensional array of beam current sensors. Inanother embodiment, the sensing device comprises a linear array of beamcurrent sensors. The flag may be translated along a horizontaltranslation path.

The angle measurement system may further comprise a processing device.The processing device may process the sensor signal to provide ameasurement of horizontal beam angle, vertical beam angle, or both. Inthe case of a two-dimensional array of beam current sensors, theprocessing device may generate a map of horizontal beam angle andvertical beam angle as a function of position in the ion beam. Thehorizontal and vertical beam angle measurements from individual beamcurrent sensors may be averaged. For example, horizontal and verticalbeam angle measurements may be averaged along a vertical column of beamcurrent sensors to provide average vertical and horizontal beam anglesat a given X coordinate. Horizontal and vertical beam angle measurementsmay be averaged over the entire array to provide overall averagehorizontal and beam angle measurements.

The measured beam angles may be compared with a predetermined criteria.If the measured beam angles meet the predetermined criteria, ionimplantation may proceed. If the measured beam angles do not meet thepredetermined criteria, the beam angles may be adjusted, or a substratemay be tilted relative to the ion beam.

According to a second aspect of the invention, a method is provided formeasuring an incidence angle of an ion beam on a substrate plane. Themethod comprises providing a flag defining a first feature and a secondfeature, wherein the second feature has a variable spacing from thefirst feature as a function of location on the second feature,translating the flag along a translation path so that the flagintercepts at least a portion of the ion beam, and detecting the ionbeam for different flag positions along the translation path andproducing in response to the detected ion beam a sensor signal having afirst signal component representative of the first feature and a secondsignal component representative of the second feature. The first andsecond signal components and corresponding positions of the flag arerepresentative of an angle of the ion beam in a direction orthogonal tothe translation path.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the accompanying drawings, which are incorporated herein by referenceand in which:

FIG. 1 is a simplified schematic diagram of an ion implanterincorporating an angle measurement system in accordance with anembodiment of the invention;

FIG. 2 is a schematic diagram of an angle measurement system inaccordance with an embodiment of the invention;

FIG. 3 is a perspective view of the angle measurement system of FIG. 2;

FIG. 4 is a front view of an embodiment of a multi-pixel sensor arrayused in the angle measurement system of FIG. 2;

FIG. 5 illustrates a first embodiment of a flag used in the anglemeasurement system of FIG. 2;

FIG. 6 illustrates an example of a sensor signal as a function of flagposition;

FIG. 7A is a partial schematic top view of the angle measurement systemof FIG. 2;

FIG. 7B is a partial schematic side view of the angle measurement systemof FIG. 2;

FIG. 8 is a graph of sensor signal as a function of flag position fordifferent horizontal angles of the ion beam;

FIG. 9 is a graph of sensor signal as a function of flag position fordifferent vertical angles of the ion beam;

FIG. 10 illustrates a second embodiment of an a flag according to theinvention;

FIG. 11 illustrates a third embodiment of a flag according to theinvention;

FIG. 12 illustrates a fourth embodiment of a flag according to theinvention; and

FIG. 13 is a partial cross-sectional schematic diagram of the sensorarray of FIG. 4.

DETAILED DESCRIPTION

A block diagram of an embodiment of an ion implanter is shown in FIG. 1.An ion source 10 generates ions and supplies an ion beam 12. Ion source10 may include an ion chamber and a gas box containing a gas to beionized. The gas is supplied to the ion chamber where it is ionized. Theions thus formed are extracted from the ion chamber to form ion beam 12.Ion beam 12 is directed between the poles of a resolving magnet 32. Afirst power supply 14 is connected to an extraction electrode of ionsource 10 and provides a positive first voltage V₀. First voltage V_(o)may be adjustable, for example, from about 0.2 to a 80 kV. Thus, ionsfrom ion source 10 are accelerated to energies of about 0.2 to 80 KeV bythe first voltage V₀.

Ion beam 12 passes through a suppression electrode 20 and a groundelectrode 22 to a mass analyzer 30. The mass analyzer 30 includesresolving magnet 32 and a masking electrode 34 having a resolvingaperture 36. Resolving magnet 32 deflects ions in ion beam 12 such thations of a desired ion species pass through resolving aperture 36 andundesired ion species do not pass through resolving aperture 36 but areblocked by the masking electrode 34. In one embodiment, resolving magnet32 deflects ions of the desired species by 90°.

Ions of the desired ion species pass through resolving aperture 36 to afirst deceleration stage 50 positioned downstream of mass analyzer 30.Deceleration stage 50 may include an upstream electrode 52, asuppression electrode 54 and a downstream electrode 56. Ions in the ionbeam may be decelerated by deceleration stage 50 and then pass throughan angle corrector magnet 60. Angle corrector magnet 60 deflects ions ofthe desired ion species and converts the ion beam from a diverging ionbeam to a ribbon ion beam 62 having substantially parallel iontrajectories. In one embodiment, angle corrector magnet 60 deflects ionsof the desired ion species by 70°.

An end station 70 supports one or more semiconductor wafers, such aswafer 72, in the path of ribbon ion beam 62 such that ions of thedesired species are implanted into the semiconductor wafer. The endstation 70 may include a cooled electrostatic platen and a scanner (notshown) for moving wafer 72 perpendicular to the long dimension of theribbon ion beam 62 cross-section, so as to distribute ions over thesurface of wafer 72. The ribbon ion beam may be at least as wide aswafer 72.

The ion implanter may include a second deceleration stage 80 positioneddownstream of angle corrector magnet 60. Deceleration stage 80 mayinclude an upstream electrode 82, a suppression electrode 84 and adownstream electrode 86.

The ion implanter may include additional components known to thoseskilled in the art. For example, end station 70 typically includesautomated wafer handling equipment for introducing wafers into the ionimplanter and for removing wafers after ion implantation. End station 70may also include a dose measuring system, an electron flood gun andother known components. It will be understood that the entire pathtraversed by the ion beam is evacuated during ion implantation.

The ion implanter of FIG. 1 may operate in one of several modes. In afirst operating mode, known as the drift mode, deceleration stages 50and 80 are connected to ground, and the ion beam 12 is transportedthrough the beamline at the final beam energy established afterextraction from ion source 10. In a second operating mode, known as theenhanced drift mode, the ion beam 12 is accelerated to an intermediateenergy at electrode 22 before passing through mass analyzer 30 and thenis decelerated to the final beam energy by first deceleration stage 50.In a third operating mode, known as the double deceleration mode, theion beam is accelerated to a first intermediate energy at electrode 22before passing through mass analyzer 30, is decelerated by firstdeceleration stage 50 to a second intermediate energy as it passesthrough angle corrector 60 and then is decelerated to the final beamenergy by second deceleration stage 80. A fourth operating modetransports the beam at the intermediate energy through to the seconddeceleration stage 80, and the gap at the first deceleration stage 50 isoperated with a short circuit shunt. By transporting the ion beamthrough part of the beamline at higher energy, space charge expansioncan be reduced in comparison with the drift mode for a given final beamenergy.

In accordance with an aspect of the invention, end station 70 mayinclude an angle measurement system 100 as shown in FIG. 1. Anglemeasurement system 100 is configured to measure ion beam angles in oneor both of two orthogonal directions. Typically, ion beam angles withrespect to a substrate plane 110 are of interest. However, anglemeasurement system 100 can measure ion beam angles with respect to anydesired plane. It is useful to define a coordinate system wherein theorigin is at the center of a wafer positioned for ion implantation insubstrate plane 110, the X axis is horizontal and in substrate plane110, the Y axis is vertical and in substrate plane 110, and the Z axisis perpendicular to substrate plane 110.

A block diagram of angle measurement system 100 in accordance with anembodiment of the invention is shown in FIG. 2. A perspective view ofangle measurement system 100 is shown in FIG. 3. Angle measurementsystem 100 includes a flag 120, a translation mechanism 122 to translateflag 120 in the X direction, and a sensing device 123 positioneddownstream in the Z direction from flag 120. In the embodiment of FIGS.2 and 3, sensing device 123 is a multi-pixel sensor array 124. Aprocessor 130 stores and/or processes sensor signals produced by sensorarray 124.

Flag 120 may be a plate that includes a first feature 140 and a secondfeature 142, which permit ion beam angles to be measured as describedbelow. Features 140 and 142 are described below in connection with FIG.5. Flag 120 selectively blocks ion beam 62 as described below.

Translation mechanism 122 may include an actuator 150 coupled to flag120 by a connecting rod 152 and a support block 154. Translationmechanism 122 translates flag 120 along a translation path 170 in the Xdirection in substrate plane 110.

Multi-pixel sensor array 124 is spaced from substrate plane 110 in the Zdirection, and in the embodiment of FIG. 2, includes a two-dimensionalarray of ion beam sensors, such as Faraday cup beam current sensors.Multi-pixel sensor array 124 senses ribbon ion beam 62 for differentflag positions along translation path 170. Multi-pixel sensor array 124provides sensor signals in response to the sensed beam current. Thesensor signals are supplied to processor 130 for processing to determineion beam angles as described below. Processor 130 may supply positioncontrol signals to actuator 150.

Referring to FIG. 4, multi-pixel sensor array 124 may include multiplebeam current sensors 162 mounted to a housing 164. Each of beam currentsensors 162 may be a Faraday cup which produces an electrical signal inresponse to an intercepted ion beam. As known in the art, the magnitudeof the sensor signal is a function of the intercepted ion beam current.Each beam current sensor may be a cup-shaped conductor with an aperturethat faces the ion beam. The size of the aperture determines the area ofthe ion beam that is sampled by the beam current sensor. In oneembodiment, 9 millimeter square apertures are utilized.

In the embodiment of FIG. 4, sensor array 124 is a two-dimensional arrayof beam current sensors 162 including seven spaced-apart columns 166 ofbeam current sensors. Individual beam current sensors within each columnare equally spaced in the Y direction, and columns 166 are equallyspaced in the X direction. Adjacent columns are offset in the Ydirection to avoid measurement gaps.

In other embodiments, sensing device 123 may include a single beamcurrent sensor, a linear array of beam current sensors, or an arbitraryarrangement of beam current sensors. As described below, each individualbeam current sensor can be used to measure horizontal beam angle,vertical beam angle, or both at the position of each beam currentsensor. A sensing device 123 having two or more beam current sensors,such as an array of beam current sensors, may be used to measurehorizontal beam angle, vertical angle, or both at two or more positionsin substrate plane 110. The arrangement of beam current sensors dependson the beam angle information required for a particular application.Thus, sensing device 123 may include beam current sensors at selectedpositions of interest relative to substrate plane 110.

A first embodiment of flag 120 is shown in FIG. 5. In FIG. 5, flag 120is viewed in the negative Z direction, i.e. the upstream direction ofthe ion beam. Flag 120 may be a plate having features which permitmeasurement of ion beam angles. The plate may be fabricated of graphite,for example.

In the embodiment of FIG. 5, first feature 140 is a vertical slot 144having a long dimension parallel to the Y direction, and second feature142 is an angled edge 146 of flag 120. Preferably, the edges of theplate which define slot 144 and angled edge 146 are beveled to limitsputtering by the ion beam. Flag 120 may be any element that blocks theion beam and includes features that permit ion beam angles to bemeasured as described herein.

Second feature 142 has a variable spacing from first feature 140 as afunction of location on second feature 142. In the embodiment of FIG. 5,angled edge 146 has a variable spacing from slot 144 as a function oflocation on edge 146. Thus, for example, edge 146 and slot 144 have aspacing S1 at location 146 a on edge 146; edge 146 and slot 144 have aspacing S2 at location 146 b on edge 146; and edge 146 and slot 144 havea spacing S3 at location 146 c on edge 146. The first and secondfeatures 140 and 142 may be straight, but are not required to bestraight. In one embodiment, first and second features 140 and 142 areboth straight, and one of the features is orthogonal to the direction oftranslation of flag 120. The first and second features 140 and 142 maybe oriented at an acute angle Ω. Preferably, the angle Ω between thefirst and second features is in a range of about 20 to 45 degrees.

As discussed below, different embodiments of flag 120 may be utilized.Each feature of flag 120 is characterized by an edge or a slot thatprovides a transition between blocking the ion beam and passing the ionbeam. The arrangement of features 140 and 142 permits horizontal andvertical beam angles to be measured. As used herein, a horizontal beamangle is a beam angle in an X–Z plane, and a vertical beam angle is abeam angle in a Y–Z plane.

In operation, flag 120 is translated along translation path 170 (FIG. 2)in the X direction by translation mechanism 122 so that flag 120intercepts ion beam 62 along at least part of translation path 170.Typically, flag 120 is translated across the entire width of ion beam 62or, in the case of a scanned ion beam, across the entire scan width.Beam current measurements are acquired from beam current sensors 162 inmulti-pixel array 124 for different flag positions along the translationpath 170. The beam current measurements may be acquired when flag 120 ismoving or, in the case of stepwise movement, each time flag 120 stopsalong the translation path. The sensor signals generated by beam currentsensors 162 may be measured continuously or may be sampled at desiredintervals. The current measurements are supplied to processor 130 forstorage in a memory. The set of current measurements for differentpositions of flag 120 represents ion beam angles in two dimensions. Theset of current measurements may be processed to provide angle values asdiscussed below.

The parameters of angle measurement system 100 depend on thecharacteristics of ion beam 62, such as beam current and cross-sectionaldimensions and shape, and on the desired angle measurement resolutionand measuring speed. The height and width of multi-pixel sensor array124 should be greater than the maximum expected height and width of ionbeam 62 in order to intercept beams that are displaced from a nominallocation. The size of beam current sensors 162 in array 160 depends onthe desired resolution of angle measurement and on the ability of thesensor to produce an acceptable signal level. The height of flag 120 inthe Y direction should be at least as great as the height of sensorarray 124.

The translation of flag 120 along translation path 170 may be continuousor in discrete steps. In one embodiment, flag 120 is translated in stepsequal to one-half the width of the apertures in beam current sensors162. The translation mechanism 122 may utilize a rack and pinion drivemechanism, for example. Other suitable translation mechanisms include aball and screw assembly, a linear motor and an air piston.

Translation flag 120 along the X direction is described herein. In otherembodiments, flag 120 can be translated along the Y direction or alongan arbitrary direction that causes flag 120 to intercept at least aportion of the ion beam.

As shown in FIG. 2, processor 130 may provide position control signalsto actuator 150 to control translation of flag 120 along translationpath 170. For example, processor 130 may control actuator 150 totranslate flag 120 in steps across ion beam 62 and to record the beamcurrent sensed by each of the beam current sensors 162 in themulti-pixel array 124 at each position of flag 120. The measured currentvalues and the corresponding positions of flag 120 form a data set whichrepresents beam angles at different locations in ion beam 62. The dataset may be stored by processor 130.

An example of a sensor signal 180 as flag 120 is translated alongtranslation path 170 is shown in FIG. 6. The sensor signal 180 isplotted as a function of the position of flag 120 along translation path170. Sensor signal 180 may represent the signal generated by a beamcurrent sensor 162 a at a location (x_(px), y_(px)) in sensor array 124.Flag 120 may move from a negative value of X in the positive Xdirection. Initially, flag 120 does not block ion beam 62 from reachingcurrent sensor 162 a, and beam current sensor 162 a produces a sensorsignal of maximum value 184. As flag 120 moves in the positive Xdirection, angled edge 146 blocks ion beam 62 from reaching currentsensor 162 a, and sensor signal 180 decreases from maximum value 184 tozero, as indicated by transition 186. As flag 120 moves farther in thepositive X direction, ion beam 62 remains blocked by flag 120 fromreaching current sensor 162 a until a portion of ion beam 62, known as abeamlet 190, that passes through slot 144 is intercepted by beam currentsensor 162 a. Beamlet 190 may be defined a portion of ion beam 62 thatis intercepted by a beam current sensor 162, such as a beam currentsensor 162 a, in sensor array 124. Beamlet 190 may be formed by ionshaving a range of angles, depending on the characteristics of ion beam62. Beam current sensor 162 a produces a “bump” 188 in sensor signal 180at the X position of flag 120 where beamlet 190 passes through slot 144and is intercepted by beam current sensor 162 a. Bump 188 is a peak insensor signal 180 that depends on the width of slot 144, the aperture ofbeam current sensor 162 a, and the beam angle spread. As flag 120continues in the positive X direction, sensor signal 180 remains atzero. It will be understood that flag 120 can be translated in thenegative X direction and that the same sensor signal is produced.

Bump 188 is a first component of sensor signal 180, which isrepresentative of the first feature 140 (slot 144) of flag 120, andtransition 186 is a second component of sensor signal 180, which isrepresentative of the second feature 142 (angled edge 146) of flag 120.As described below, the X position of bump 188 indicates the horizontalangle of ion beam 62, and the spacing S between transition 186 and bump188 represents the vertical angle of ion beam 62.

Measurement of horizontal beam angle θ_(h) is described with referenceto FIGS. 5, 6, 7 A and 8. A schematic top view of the angle measurementsystem is shown in FIG. 7A. Beam current sensor 162 a is shownintercepting ion beam 62 for different horizontal beam angles. As shownin FIG. 7A, the horizontal beam angle θ_(h) is the angle between the Zdirection and ion beam 62 in a horizontal (X–Z) plane. Beamlet 190passes through slot 144 in flag 120 and is intercepted by beam currentsensor 162 a in sensor array 124. The beam current sensor 162 a has an Xcoordinate x_(px), and slot 144 has an X coordinate x_(bc) when beamlet190 is intercepted by beam current sensor 162 a. Sensor array 124 isspaced from flag 120 in the Z direction by a distance z_(px). For thisgeometry, horizontal beam angle θ_(h) may be expressed as:tan θ_(h)=(x _(px) −x _(bc))/z _(px)  (1)

The X coordinate x_(bc) of slot 144 at which beamlet 190 is interceptedby beam current sensor 162 a varies as the horizontal beam angle varies.Thus, for an example of an ion beam having zero horizontal beam angle,beamlet 190 is transported along a trajectory 192 to beam current sensor162 a. For an ion beam having a negative horizontal beam angle, beamlet190 is transported along a trajectory 194 to beam current sensor 162 a;and for an ion beam having a positive horizontal beam angle, beamlet 190is transported along a trajectory 196 to beam current sensor 162 a. Thedirection of the displacement relative to the beam current sensorindicates the sign of the horizontal beam angle.

Referring to FIG. 8, sensor signals generated by beam current sensor 182are shown for different horizontal beam angles θ_(h), with the verticalangle remaining constant. Sensor signal 200 represents an intermediatevalue of horizontal beam angle, sensor signal 202 represents a smallervalue of horizontal beam angle, and sensor signal 204 represents alarger value of horizontal beam angle. In each case, the X position ofthe bump, such as bump 200 a, in the sensor signal represents the valueof the horizontal beam angle. The X positions of the bumps in sensorsignals 200, 202 and 204 correspond to the X coordinate of slot 144 whenbeamlet 190 is intercepted by beam current sensor 162 a. It may beobserved that the transitions in the sensor signals due to edge 142 ofslot 120, such as transition 200 b, move by the same amount as therespective bumps in the case where the vertical angle remains constant.

Referring to FIG. 6, the X position of bump 188 represents the Xcoordinate x_(bc) of slot 144 in flag 120 when beamlet 190 passesthrough slot 144 and is intercepted by beam current sensor 162 a at Zcoordinate x_(px). Since the X coordinate x_(px) and the Z coordinatez_(px) of beam current sensor 162 a are known and fixed, and the Xcoordinate x_(bc) of slot 144 is determined from the position of bump188, the horizontal beam angle θ_(h) can be determined from equation (1)above. It will be understood that beam current sensors 162 in differentcolumns of sensor array 124 have different X coordinates x_(px). Thecenter of bump 188 is used to determine the X coordinate of beamlet 190.The center of bump 188 is defined as the point where one half of theintegrated current is on either side of the bump. This location isphysically where the slot center has crossed the centroid of the beamletarea at the wafer plane.

Measurement of vertical beam angle θ_(v) is described with reference toFIGS. 5, 6, 7B, and 9. A schematic side view of the angle measurementsystem is shown in FIG. 7B. Beam current sensor 162 a is shownintercepting beamlet 190 for different vertical beam angles. As shown inFIG. 7B, vertical beam angle θ_(v) is the angle between the Z directionand the ion beam in a vertical (Y–Z) plane. Angled edge 146 of flag 120is shown in FIG. 7B. Beamlet 190 with zero vertical beam angle istransported along a trajectory 220 to beam current sensor 162 a andintercepts edge 146 of flag 120 at location 146 b. Beamlet 190 with anegative vertical beam angle is transported along a trajectory 222 tobeam current sensor 162 a and intercepts edge 146 of flag 120 atlocation 146 a. Beamlet 190 with a positive vertical beam angle istransported along a trajectory 224 to beam current sensor 162 a andintercepts edge 146 of flag 120 at location 146 c.

As shown in FIG. 5, locations 146 a, 146 b, and 146 c correspond tospacings S1, S2, and S3, respectively, from slot 144. The spacingbetween a location on edge 146 and slot 144 is indicated in the sensorsignal 180 shown in FIG. 6 by a spacing S between bump 188 andtransition 186. Thus, a zero vertical beam angle θ_(v) is indicated by aspacing S in sensor signal 180 corresponding to spacing S2, for example,in FIG. 5. Similarly, a positive vertical beam angle is indicated insensor signal 180 by a smaller value of spacing S corresponding tospacing S1, for example, in FIG. 5; and a negative vertical beam angleis indicated in sensor signal 180 by a larger value of spacing Scorresponding to spacing S3 in FIG. 5, for example. The spacing Sbetween bump 188 and transition 186 defines the location on edge 146where beamlet 190 was crossed and is used to determine the Y coordinateof beamlet 190. The X coordinate of bump 188 is determined as describedabove. The center of edge 146 is determined as the position where onehalf of the sensor signal occurs. This position is physically where edge146 of flag 120 has shadowed one half of the beamlet area at the waferplane.

Examples of sensor signals for different values of vertical beam angleθ_(v) are shown in FIG. 9. In FIG. 9, the horizontal beam angle θ_(h) isassumed to be constant. A sensor signal 300 may correspond to zerovertical beam angle, a sensor signal 302 may correspond to a negativevertical beam angle, and a sensor signal 304 may correspond to apositive vertical beam angle. Each sensor signal includes a bump, suchas bump 300 a, representative of slot 144 in flag 120 and a transition,such as transition 300 b, representative of edge 146 of flag 120. The Xposition of bump 300 a remains fixed for a constant horizontal beamangle. In each case, the spacing between bump 300 a and the transitionrepresentative of edge 146 corresponds to the value of the vertical beamangle. Thus, a relatively small spacing, such as spacing S1, correspondsto a negative vertical beam angle, and a relatively large spacing, suchas spacing S3, corresponds to a positive vertical beam angle.

As shown in FIG. 7B, beam current sensor 162 a has a Y coordinatey_(py), and the ion beam intersects edge 146 at Y coordinate y_(bc).Beam current sensor 162 a is spaced from flag 120 in the Z direction bya distance z_(px). For this geometry, the vertical beam angle θ_(v) maybe expressed as:tan θ_(v)=(y _(px) −y _(bc))/z _(px)  (2)

The Y coordinate y_(px) and the Z coordinate z_(px) of beam currentsensor 162 a are known and fixed, and the Y coordinate y_(bc) at whichbeamlet 190 crosses edge 146 is determined from spacing S between bump188 and transition 186 in sensor signal 180. For example, the relationbetween spacing S and Y coordinate y_(bc) can be obtained from a table.Accordingly, the vertical beam angle θ_(v) can be determined fromequation (2) above. It will be understood that beam current sensors 162in different rows of sensor array 124 have different Y coordinatesy_(px).

FIGS. 6, 8, and 9 and the accompanying descriptions relate to the sensorsignals produced by a single beam current sensor 162 a in sensor array124. Beam current sensor 162 a produces a sensor signal that permits thehorizontal and vertical beam angles at one position in the ion beam tobe determined according to equations (1) and (2), respectively. Each ofthe beam current sensors in sensor array 124 produces a similar sensorsignal as a function of flag position. The horizontal beam angle and thevertical beam angle of ion beam 62 may vary as a function of position insubstrate plane 110. The sensor array 124 measures the horizontal andvertical beam angles at multiple positions in substrate plane 110. Thus,each pixel (beam current sensor 162) at location (x_(px), y_(px)) insensor array acquires a measurement of horizontal beam angle and ameasurement of vertical beam angle. For example, in the case of an ionbeam that is perfectly parallel, the beam current sensors all measurethe same horizontal and vertical beam angles. In the case of a divergingion beam, beam current sensors 162 at different locations in sensorarray 124 measure different horizontal and vertical beam angles. Themeasurements acquired by beam current sensors 162 in sensor array 124thus constitute a map of horizontal and vertical beam angles as afunction of position in the substrate plane 110.

Sensor signals from some or all of the beam current sensors 162 insensor array 124 may be processed provide additional information. Forthe case of a broad beam, either a ribbon beam or a scanned beam, beamangle changes, either horizontal or vertical, at various horizontalpositions across the beam are of interest. This is because a point onthe wafer intercepts the beam from one horizontal position within thebeam but at that horizontal position, the wafer intercepts every part ofthe beam vertically due to the vertical mechanical scan of theimplanter.

Accordingly, sensor signals from all beam current sensors at the samehorizontal location may be processed to provide a weighted average ofthe horizontal and/or vertical angle measurements to obtain the averagehorizontal and/or vertical angle at that horizontal position. The anglemeasurements for each beam current sensor may be weighted by the beamcurrent detected at that beam current sensor. For the example of a pixelarray having seven vertical columns, this operation produces sevenaverage horizontal angles and seven average vertical angles. Since theion beam is typically made uniform as a function of horizontal position,an equally weighted average of the seven measurements defines an overallaverage direction of the beam. The variation within the seven averagesmay be defined as a beam angle spread for a converging beam. The resultmay be zero horizontal beam angle in the middle, positive horizontalbeam angle on the negative side of the wafer and negative horizontalbeam angle on the positive side of the wafer.

The angle measurement system disclosed herein maybe used to measure beamangle spread as well as beam angles. Beam angle spread is the range ofangles included in ion beam 62. In the context of beamlet 190, beamangle spread is the range of angles of ions intercepted by a single beamcurrent sensor. Sensor array 124 may be used to evaluate beam anglespread at different positions in ion beam 62, with a beam angle spreadmeasurement made by each beam current sensor in sensor array 124. Eachbeam current sensor may make a measurement of horizontal beam anglespread φ_(h) and a measurement of vertical beam angle spread φ_(v).Qualitatively, horizontal beam angle spread φ_(h) is indicated by thewidth of bump 188 in sensor signal 180. Thus, a wider bump 188 isindicative of a larger horizontal beam angle spread. Similarly, verticalbeam angle spread φ_(v) is indicated by the slope of transition 186 insensor signal in 180. A more gradual transition 186 is indicative of alarger vertical beam angle spread.

Horizontal beam angle spread φ_(h) may be determined as follows.

$\begin{matrix}{\varphi_{n} \simeq {\left( {{\Delta\; x_{b}} - {\Delta\; x_{px}}} \right)/z_{px}}} & (3) \\{\simeq {\left( {W_{b} - {2W_{s}} - {\Delta\; x_{px}}} \right)/z_{px}}} & (4)\end{matrix}$

where Δx_(b) is the beamlet window width, i.e., a range of X positionsover which beamlet 190 may pass through slot 144 and be intercepted byis beam current sensor 162 a; Δx_(px) is the X direction dimension ofthe aperture in beam current sensor 162 a; z_(px) is the Z directionspacing between flag 120 and beam current sensor 162 a; W_(b) is thewidth of the bump in sensor signal 180 measured, for example, at 0.01 ofmaximum amplitude; and W_(s) is the width of slot 144. In equation (4),bump width W_(b) is the only variable for a given configuration.Therefore, horizontal beam angle spread φ_(h) is expressed as a functionof bump width W_(b).

Vertical beam spread φ_(v), maybe determined as follows.

$\begin{matrix}{\varphi_{v} \simeq {\left( {{\Delta\; y_{b}} - {\Delta\; y_{px}}} \right)/z_{px}}} & (5) \\{\simeq {\left\lbrack {{\left( {W_{r} - {\Delta\; x_{b}}} \right)\tan\;\Omega} - {\Delta\; y_{px}}} \right\rbrack/z_{px}}} & (6)\end{matrix}$

where Δy_(b) is the beamlet window height; Δy_(px) is the Y directiondimension of the aperture in beam current sensor 162 a; W_(r) is thewidth of transition 186 measured, for example, from full amplitude to0.01 of full amplitude; Δx_(b) is the beamlet window width as describedabove; and Ω is the angle of edge 146. In equation (6), transition widthW_(r) and beamlet window width Δx_(b) (given by W_(b)−2 W_(s)) are theonly variables for a given configuration.

The beam angle measurements may be evaluated to determine possibleactions. For example, acceptable criteria may be established for beamangle parameters. If the beam angle parameters meet the acceptablecriteria, ion implantation may proceed. If the beam angle parameters donot meet the acceptable criteria, the ion implanter can be adjusted tobring the beam angle parameters within the acceptable criteria. Inanother approach, the wafer can be tilted as described in U.S. Pat. No.6,437,350, which is hereby incorporated by reference, to bring the beamangle parameters within the acceptable criteria.

Other suitable flag configurations are shown in FIGS. 10–12. As shown inFIG. 10, a flag 400 includes a first feature in the form of a verticaloutside edge 402 and a second feature in the form of an angled slot 404.In the embodiment of FIG. 10, the position of the bump in the sensorsignal resulting from slot 404 moves as a function of vertical beamangle and the position of the transition produced by edge 402 moves as afunction of horizontal beam angle.

As shown in FIG. 11, a flag 420 includes a first feature in the form ofa vertical edge 424 and a second feature in the form of an angled edge426, both of which are inside edges of an aperture 422 in flag 420.Vertical edge 424 produces a transition in the sensor signal that isused to determine horizontal beam angle, and angled edge 426 produces atransition in the sensor signal that is used to measure vertical beamangle.

As shown in FIG. 12, a flag 440 includes a first feature in the form ofa vertical edge 444 and a second feature in the form of an angled edge446, both of which are outside edges of flag 440. Vertical edge 444produces a transition in the sensor signal that is used to measurehorizontal beam angle, and angled edge 446 produces a transition in thesensor signal that is used to measure vertical beam angle. Flag 440 maybe supported by a post 448 for translation along a translation path.

Different ion beam types can be measured by the angle measurement systemdisclosed herein. A ribbon ion beam typically has an elongatedcross-section characterized by a beam height and a beam width. Theheight of the sensor array 124 in the Y direction is selected to begreater than the beam height to permit beam angle measurement at themaximum expected vertical beam angle. The length of translation path 170in the X direction is selected to be equal to or greater than the beamwidth. It is most practical to translate the flag 120 along the longdimension of the ion beam cross-section. However, the invention is notlimited in this regard.

The angle measurement system of the present invention can be utilized toacquire angle measurements of a scanned ion beam. The ion beam isscanned in a scan direction to provide a scan pattern. The anglemeasurement system can be utilized to obtain beam angle measurementsover the area of the scan pattern. The flag 120 is translated in thescan direction, and the translation path 170 has a length equal to orgreater than the scan pattern. The speed of translation of flag 120 isslow in comparison with the beam scanning speed to ensure that the ionbeam angle is measured at least once at each position along thetranslation path.

The angle measurement system of the present invention can be utilized toacquire angle measurements of a fixed spot ion beam. The height andwidth of sensor array 124 are both greater than the diameter of the spotion beam to permit beam angle measurement at the maximum expected beamangles. The spot ion beam does not necessarily have a circularcross-section and in general has an irregular cross-sectional shape.

A partial cross-sectional schematic diagram of sensor array 124 is shownin FIG. 13. Beam current sensors 500 and 502 of sensor array 124 areshown. An aperture plate 510 having apertures 512 and 514 defines theareas of ion beam 62 that are sensed by the respective beam currentsensors. In particular, aperture 512 defines a sensor area of beamcurrent sensor 500, and aperture 514 defines a sensor area of beamcurrent sensor 502. Each aperture has dimensions (Δx_(px), Δy_(px)), asshown in FIGS. 7A and 7 B. Beam current sensors 500 and 502 furtherinclude collector cups 520 and 522, respectively, mounted to supportplate 530. Collector cups 520 and 522 are connected to ground throughcurrent sensors 524 and 526, respectively.

One of the difficulties in measuring beam angles, particularly whenmeasuring low ion beam currents, is to ensure that only the current dueto the ion beam is measured. The angle measurement system operates in aregion of the ion implanter where electrons and low energy ions are alsopresent, generated both through collisions of the ion beam with thebackground gas and by introduction from an electron flood gun used toneutralize charge on the wafer. The sensor array 124 may be providedwith suppression elements for suppressing entry of electrons and lowenergy ions into the beam current sensors.

In the embodiment of FIG. 13, sensor array 124 includes first, secondand third electrostatic suppression electrodes 540, 542 and 544positioned between aperture plate 510 and collector cups 520 and 522.Each of suppression electrodes 540, 542 and 544 includes aperturesaligned with the apertures in aperture plate 510. By way of example,aperture plate 510 can be grounded. First suppression electrode 540 andthird suppression electrode 544 may be biased at −50 to −1000 volts, andsecond suppression electrode 542 may be biased at +50 to +500 volts. Inone example, electrodes 540 and 544 are biased at −500 volts andelectrode 542 is biased at +100 volts. It will be understood that thesevalues are given by way of example only and are not limiting as to thescope of the present invention.

The embodiment of FIG. 13 includes electrostatic suppression elements.In other embodiments, magnetic suppression elements or a combination ofelectrostatic and magnetic suppression elements may be utilized. In oneexample, a single electrostatic suppression electrode may be utilized.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within and scope ofthe invention. Accordingly, the foregoing description is by way ofexample only and is not intended as limiting. The invention is limitedonly as defined in the following claims and the equivalents thereto.

1. An angle measurement system for an ion beam, comprising: a flag defining a first feature and a second feature, wherein the second feature has a variable spacing from the first feature as a function of location on the second feature; a mechanism to translate the flag along a translation path so that the flag intercepts at least a portion of the ion beam; and a sensing device to detect the ion beam for different flag positions along the translation path and to produce in response to the detected ion beam a sensor signal having a first signal component representative of the first feature and a second signal component representative of the second feature, wherein the first and second signal components and corresponding positions of the flag are representative of an angle of the ion beam in a direction orthogonal to the translation path.
 2. An angle measurement system as defined in claim 1, wherein at least one of the first and second features comprises a slot in the flag.
 3. An angle measurement system as defined in claim 1, wherein at least one of the first and second features comprises an outside edge of the flag.
 4. An angle measurement system as defined in claim 1, wherein at least one of the first and second features comprises inside edge of an aperture in the flag.
 5. An angle measurement system as defined in claim 1, wherein the first feature comprises a slot in the flag and the second feature comprises an angled edge of the flag.
 6. An angle measurement system as defined in claim 1, wherein the first and second features are straight.
 7. An angle measurement system as defined in claim 6, wherein the first and second features are oriented at an acute angle.
 8. An angle measurement system as defined in claim 1, wherein the first feature is orthogonal to the translation path.
 9. An angle measurement system as defined in claim 1, wherein the sensing device comprises one or more beam current sensors positioned to intercept the ion beam.
 10. An angle measurement system as defined in claim 1, wherein the sensing device comprises a linear array of beam current sensors.
 11. An angle measurement system as defined in claim 1, wherein the sensing device comprises a two-dimensional array of beam current sensors.
 12. An angle measurement system as defined in claim 1, further comprising a processing device responsive to the sensor signal for determining a horizontal beam angle in a horizontal plane and a vertical beam angle in a vertical plane.
 13. An angle measurement system as defined in claim 12, wherein the sensing device comprises a two-dimensional array of beam current sensors and wherein the processing device is configured to average sensor signals along a row or a column of the two-dimensional array.
 14. An angle measurement system as defined in claim 12, wherein the sensing device comprises a two-dimensional array of beam current sensors and wherein the processing device is configured to average sensor signals over the two-dimensional array.
 15. An angle measurement system as defined in claim 1, further comprising a processing device responsive to the sensor signal for determining a vertical beam angle in a vertical plane.
 16. An angle measurement system as defined in claim 1, wherein the sensing device comprises a two-dimensional array of beam current sensors, further comprising a processing device responsive to the sensor signals for determining a vertical beam angle at each sensor location in the two-dimensional array.
 17. An angle measurement system as defined in claim 1, wherein the sensing device comprises a two-dimensional array of beam current sensors, further comprising a processing device responsive to the sensor signals for determining a map of beam angle as a function of beam current sensor location in the two-dimensional array.
 18. A method for measuring an angle of an ion beam, comprising: providing a flag defining a first feature and a second feature, wherein the second feature has a variable spacing from the first feature as a function of location on the second feature; translating the flag along a translation path so that the flag intercepts at least a portion of the ion beam; and detecting the ion beam for different flag positions along the translation path and producing in response to the detected ion beam a sensor signal having a first signal component representative of the first feature and a second signal component representative of the second feature, wherein the first and second signal components and corresponding positions of the flag are representative of an angle of the ion beam in a direction orthogonal to the translation path.
 19. A method as defined in claim 18, further comprising processing the sensor signal to provide a measurement of vertical beam angle.
 20. A method as defined in claim 18, further comprising processing the sensor signal to provide a measurement of vertical beam angle and horizontal beam angle.
 21. A method as defined in claim 18, wherein detecting the ion beam comprises detecting the ion beam with a two-dimensional array of beam current sensors to provide measurements of beam angle at multiple positions in a substrate plane.
 22. A method as defined in claim 20, wherein detecting the ion beam comprises detecting the ion beam with a two-dimensional array of beam current sensors to provide measurements of vertical beam angle and horizontal beam angle at multiple positions in a substrate plane.
 23. A method as defined in claim 18, wherein detecting the ion beam comprises detecting the ion beam with a two-dimensional array of beam current sensors, further comprising processing the sensor signals along a row or a column of the two-dimensional array to provide an average value of beam angle along the row or the column.
 24. A method as defined in claim 18, wherein detecting the ion beam comprises detecting the ion beam with a two-dimensional array of beam current sensors, further comprising processing the sensor signals from the two-dimensional array of beam current sensors to provide an average value of beam angle over the ion beam.
 25. A method as defined in claim 18, wherein detecting the ion beam comprises detecting the ion beam with a two-dimensional array of beam current sensors, further comprising processing the sensor signals from the two-dimensional array of beam current sensors to provide a measurement of beam angle spread.
 26. A method as defined in claim 18, further comprising processing the sensor signal to provide a beam angle measurement and adjusting the ion beam if the measured beam angle does not meet a predetermined criteria.
 27. A method as defined in claim 18, further comprising processing the sensor signal to provide a beam angle measurement and tilting a substrate relative to the ion beam if the measured beam angle does not meet a predetermined criteria. 